Remove some cruft from the Makefile

This was affecting the build, but the files don't actually exist.

Makefile.am

@@ -51,9 +51,6 @@ glsl_SOURCES = \
 	ir_hierarchical_visitor.cpp \
 	ir_swizzle_swizzle.cpp \
 	ir_vec_index_to_swizzle.cpp
-	ir_vec_index_to_swizzle.cpp \
-	ir_visit_tree.cpp \
-	ir_visit_tree.h
 
 BUILT_SOURCES = glsl_parser.h glsl_parser.cpp glsl_lexer.cpp
 CLEANFILES = $(BUILT_SOURCES)