gallium/util: remove unused util_strbuf*
gallium/util: remove unused util_strbuf*

Looks like they have been unused since 2008 b8a7eef242f6bb97d90f.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler