gl_ClipDistance needs special treatment in form of lowering pass
which transforms gl_ClipDistance representation from float[] to
vec4[]. There are 2 implementations - at glsl linker level (enabled
by LowerClipDistance option) and at glsl_to_tgsi level (enabled
unconditionally for gallium drivers). Second implementation is
incomplete - it does not take into account transform feedback (see
commit 642e5b41 "mesa: Fix transform
feedback of unsubscripted gl_ClipDistance array" for details).

There are 2 possible fixes:
- adding transform feedback support into glsl_to_tgsi version
- ripping gl_ClipDistance support from glsl_to_tgsi and enabling
  gl_ClipDistance lowering on glsl linker side

This patch implements 2nd option. All it does is:
- reverts most of the commit 59be6916
  "st/mesa: add support for gl_ClipDistance"
- changes LowerClipDistance to true

Fixes Piglit tests "EXT_transform_feedback/builtin-varyings
gl_ClipDistance[{2,3,4,5,6,7,8}]-no-subscript" at least on nv50
and evergreen cards.