GLES:Remove gstate_c.textureFullAlpha check

Fix Super Danganronp​a 2 butterfly colour issue
#5996
before
![1](https://cloud.githubusercontent.com/assets/2177532/2857295/09834970-d176-11e3-8365-0d5a972e00b8.jpg)
after
![2](https://cloud.githubusercontent.com/assets/2177532/2857299/0ea851f2-d176-11e3-8c3d-72c5fb359c8b.jpg)

This fix follow comment in https://github.com/hrydgard/ppsspp/commit/f33ddad364509b681494e3862d500dd9f7db2daa