Remove outdated proguard rules for Retrofit

[Goooler]

Follow up #267.

https://github.com/square/retrofit/blob/6b6984e9dd683bff6cdb5931694e1b8efc3533ae/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro#L12-L14

[Goooler]

OLD: old.apk (signature: V1, V2, V3)
NEW: new.apk (signature: V1, V2, V3)

          │          compressed           │          uncompressed
          ├───────────┬───────────┬───────┼───────────┬───────────┬────────
 APK      │ old       │ new       │ diff  │ old       │ new       │ diff
──────────┼───────────┼───────────┼───────┼───────────┼───────────┼────────
      dex │   1.7 MiB │   1.7 MiB │ -99 B │   3.7 MiB │   3.7 MiB │ -288 B
     arsc │ 301.8 KiB │ 301.8 KiB │   0 B │ 301.7 KiB │ 301.7 KiB │    0 B
 manifest │   2.8 KiB │   2.8 KiB │   0 B │  10.9 KiB │  10.9 KiB │    0 B
      res │ 267.4 KiB │ 267.4 KiB │  +4 B │ 381.6 KiB │ 381.6 KiB │    0 B
    asset │   5.3 KiB │   5.3 KiB │  +8 B │   5.1 KiB │   5.1 KiB │   +2 B
    other │  28.8 KiB │  28.8 KiB │  +9 B │  58.7 KiB │  58.7 KiB │    0 B
──────────┼───────────┼───────────┼───────┼───────────┼───────────┼────────
    total │   2.3 MiB │   2.3 MiB │ -78 B │   4.4 MiB │   4.4 MiB │ -286 B


 DEX     │ old   │ new   │ diff
─────────┼───────┼───────┼────────────
   files │     1 │     1 │  0
 strings │ 24883 │ 24881 │ -2 (+0 -2)
   types │  5273 │  5272 │ -1 (+0 -1)
 classes │  4164 │  4164 │  0 (+0 -0)
 methods │ 24722 │ 24722 │  0 (+0 -0)
  fields │ 26993 │ 26992 │ -1 (+0 -1)


 ARSC    │ old  │ new  │ diff
─────────┼──────┼──────┼──────
 configs │   50 │   50 │  0
 entries │ 1376 │ 1376 │  0


=================
====   APK   ====
=================

    compressed    │   uncompressed    │
──────────┬───────┼──────────┬────────┤
 size     │ diff  │ size     │ diff   │ path
──────────┼───────┼──────────┼────────┼────────────────────────────────
  1.7 MiB │ -99 B │  3.7 MiB │ -288 B │ ∆ classes.dex
 13.5 KiB │ +14 B │ 28.4 KiB │    0 B │ ∆ META-INF/CERT.SF
    622 B │  +7 B │    484 B │   +1 B │ ∆ assets/dexopt/baseline.profm
   13 KiB │  -6 B │ 28.3 KiB │    0 B │ ∆ META-INF/MANIFEST.MF
    313 B │  +4 B │    207 B │    0 B │ ∆ res/0H.9.png
  4.7 KiB │  +1 B │  4.6 KiB │   +1 B │ ∆ assets/dexopt/baseline.prof
  1.2 KiB │  +1 B │  1.3 KiB │    0 B │ ∆ META-INF/CERT.RSA
──────────┼───────┼──────────┼────────┼────────────────────────────────
  1.7 MiB │ -78 B │  3.7 MiB │ -286 B │ (total)



=================
====   DEX   ====
=================

STRINGS:

   old   │ new   │ diff
  ───────┼───────┼────────────
   24883 │ 24881 │ -2 (+0 -2)

  - Ljava/lang/annotation/Documented;
  - PARAMETER


TYPES:

   old  │ new  │ diff
  ──────┼──────┼────────────
   5273 │ 5272 │ -1 (+0 -1)

  - Ljava/lang/annotation/Documented;


FIELDS:

   old   │ new   │ diff
  ───────┼───────┼────────────
   26993 │ 26992 │ -1 (+0 -1)

  - java.lang.annotation.ElementType PARAMETER: ElementType