gallium: remove PIPE_TEX_FILTER_ANISO

gallium: remove PIPE_TEX_FILTER_ANISO

This patch removes PIPE_TEX_FILTER_ANISO.

Anisotropic filtering is enabled if and only if max_anisotropy > 1.0.
Values between 0.0 and 1.0, inclusive, of max_anisotropy are to be
considered equivalent, and meaning to turn off anisotropic filtering.

This approach has the small drawback of eliminating the possibility of
enabling anisotropic filter on either minification or magnification
separately, which Radeon hardware seems to support, is currently
support by Gallium but not exposed to OpenGL.  If this is actually
useful it could be handled by splitting max_anisotropy in two values
and adding an appropriate OpenGL extension.

NOTE: some fiddling & reformatting by keithw to get this patch to
apply.  Hopefully nothing broken in the process.