mesa: Remove unnecessary and incorrect TexEnv parameter validation.

mesa: Remove unnecessary and incorrect TexEnv parameter validation.

For GL_RGB_SCALE and GL_ALPHA_SCALE targets, the API wrapper code
attempts to ensure the parameter is 1.0, 2.0, or 4.0.

This is unnecessary: set_combiner_scale in texenv.c (called by
_mesa_TexEnvfv) already checks this and raises an appropriate error.

It's also incorrect: For glTexEnvx, the API validation code directly
compares the GLfixed input parameter with a floating point constant,
prior to converting fixed-point to floating point.

Fixes an issue in the OpenGL ES 1.1 conformance suite.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>