Remove ref-names from .git_archival.txt (#444)

It turns out that the `ref-names` field in `.git_archival.txt` will generate a different value depending on whether the commit an archive was built from was the head of a branch at the time of archiving. That's a problem because it means someone trying to compare the hash signatures of two archives built from the same commit might get different results based on *when* the archives were made, and not on any other intrinsic feature of the archives' content. This removes the field in order to make Git archives a bit more stable and useful.

.git_archival.txt

@@ -1,4 +1,3 @@
 node: $Format:%H$
 node-date: $Format:%cI$
 describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
-ref-names: $Format:%D$

docs/pages/guides/packaging_classic.md

@@ -222,7 +222,6 @@ You should also add these two files:
 node: $Format:%H$
 node-date: $Format:%cI$
 describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
-ref-names: $Format:%D$
 ```
 
 And `.gitattributes` (or add this line if you are already using this file):

docs/pages/guides/packaging_simple.md

@@ -145,7 +145,6 @@ You should also add these two files:
 node: $Format:%H$
 node-date: $Format:%cI$
 describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
-ref-names: $Format:%D$
 ```
 
 And `.gitattributes` (or add this line if you are already using this file):

{{cookiecutter.project_name}}/.git_archival.txt

@@ -1,4 +1,3 @@
 node: $Format:%H$
 node-date: $Format:%cI$
 describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
-ref-names: $Format:%D$