Remove M4 export to Make

This is now handled by a prebuilt_build_tool entry.

Test: treehugger
Change-Id: Ia746d5b274a897db0cd9e6ddac5f8923bca8e52c

cc/makevars.go

@@ -148,8 +148,6 @@ func makeVarsProvider(ctx android.MakeVarsContext) {
 	ctx.Strict("AIDL_CPP", "${aidlCmd}")
 	ctx.Strict("ALLOWED_MANUAL_INTERFACE_PATHS", strings.Join(allowedManualInterfacePaths, " "))
 
-	ctx.Strict("M4", "${m4Cmd}")
-
 	ctx.Strict("RS_GLOBAL_INCLUDES", "${config.RsGlobalIncludes}")
 
 	ctx.Strict("SOONG_STRIP_PATH", "${stripPath}")