doxygen: Remove src/mesa/shader/ references

Mesa has not had a src/mesa/shader/ folder since Mesa 7.9 removed it
in October 2010, as part of a revised GLSL compiler written by Intel.

Remove doxygen/shader.doxy and consequential changes made throughout.

In addition to removing an unnecessary Doxygen doxyfile, this change also
avoids a bug in the consolidated Doxygen output caused by
doxygen/shader.doxy inadvertently overwriting doxygen/swrast.tag via its
GENERATE_TAGFILE setting.

This bug depended upon the specific order each *.tag was built.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>

doxygen/tnl.doxy

@@ -43,7 +43,6 @@ SKIP_FUNCTION_MACROS   = YES
 TAGFILES               = tnl_dd.tag=../tnl \
                          main.tag=../core \
                          math.tag=../math \
-                         shader.tag=../shader \
                          swrast.tag=../swrast \
                          swrast_setup.tag=swrast_setup \
                          vbo.tag=vbo