Remove _mesa_memset in favor of plain memset.

This may break the SUNOS4 build, but it's no longer relevant.
diff --git a/src/mesa/shader/prog_parameter.c b/src/mesa/shader/prog_parameter.c
index 435e6ce..1feb7bd 100644
--- a/src/mesa/shader/prog_parameter.c
+++ b/src/mesa/shader/prog_parameter.c
@@ -146,8 +146,8 @@
 
       paramList->NumParameters = oldNum + sz4;
 
-      _mesa_memset(&paramList->Parameters[oldNum], 0, 
-		   sz4 * sizeof(struct gl_program_parameter));
+      memset(&paramList->Parameters[oldNum], 0,
+             sz4 * sizeof(struct gl_program_parameter));
 
       for (i = 0; i < sz4; i++) {
          struct gl_program_parameter *p = paramList->Parameters + oldNum + i;