mesa: Remove broken assertion about enabled texture targets.

mesa: Remove broken assertion about enabled texture targets.

For GLSL programs, enabledTargets can have more than one bit set.  For
example, a shader that uses sampler2D and samplerCube uniforms will have
both TEXTURE_2D_BIT and TEXTURE_CUBE_BIT set.

The code that sets _ReallyEnabled already handles this, selecting the
"highest priority" texture target.  We should simply use that.

Fixes new Piglit test incomplete-textures-of-multiple-types.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62698
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>