Remove redundant hardcoded code to tile corner rendering

[BenCheung0422]

Resolves #700.
This restriction exists for both GrassTile and SandTile.
Related investigation to the effect is done previously:

The effects when there are tiles exist on orthogonal directions (for each pair of orthogonal directions):

flowchart LR
    A1[corner] --> B11[diagonal]
    A1 --> B12[!diagonal]
    A2[!corner] --> B21[diagonal]
    A2 --> B22[!diagonal]
    B11 --> single
    B21 --> single
    B12 --> corner
    B22 --> full
    B12 -.-> B11
    B22 -.-> B21

Loading

single: singleton texture with connective; takes place only on corner parts; using "full" over "border center"
corner: whether corner texture is set
diagonal: whether diagonal tile exists
dotted arrow: effect of !side code

Source: #700 (comment)

This part of code has existed already before the last big update to the resource pack system. It was probably aiming to resolve the issue that a missing of corner might cause rendering misfunctioning. In the past, it actually functioned as like having checkCorners == false, by referring to 34c4381. What I can see is that, that method overwrite is kind of pointless as the constructions of both SandTile and GrassTile already made checkCorners = false. However, after deeper investigation, the method overwrite had been implemented before checkCorners (as you can see in 8881490). It is possibly because the developers had not realized the overshadowed functionality of code when adding checkCorners. Anyway, this redundant part of code which is as well limiting the customizability of resources is eventually removed here.