gles2: Remove the special wrapper for glBindTexture

gles2: Remove the special wrapper for glBindTexture

Previously the GLES2 backend needed a special wrapper for
glBindTexture because it needed to know the internal GL format of the
texture in order to correctly implement the GL_MODULATE texture env
mode. When GL_MODULATE is used then the RGB values are taken from the
previous texture layer rather than being fetched from the
texture. However since the material API was added Cogl no longer uses
the GL_MODULATE texture env mode but instead always uses GL_COMBINE.

Compiling the GLES2 backend broke since the more-texture-backends
branch merge because the cogl_get_internal_gl_format function was
removed and there was one place in GLES2 specific code that was using
this to bind the texture.