Revert "Fix absolute paths in clang debug output"

This reverts commit 253cab89ee. It is no
longer necessary.

Bug: 36463318
Bug: 72408185
Test: compare output directories after `m libc`
Test: compare treehugger output files
Change-Id: Ib5d7bbc13fa6f547e658247559153c61387e3604

cc/config/clang.go

@@ -116,10 +116,6 @@ func init() {
 
 		// http://b/68236239 Allow 0/NULL instead of using nullptr everywhere.
 		"-Wno-zero-as-null-pointer-constant",
-
-		// http://b/36463318 Clang executes with an absolute path, so clang-provided
-		// headers are now absolute.
-		"-fdebug-prefix-map=$$PWD/=",
 	}, " "))
 
 	pctx.StaticVariable("ClangExtraCppflags", strings.Join([]string{