Bug 1571631 - Remove MOZ_MUST_USE definition. r=sg

Depends on D108345

Differential Revision: https://phabricator.services.mozilla.com/D108346

mfbt/Attributes.h

@@ -360,43 +360,6 @@
 #  define MOZ_INFALLIBLE_ALLOCATOR
 #endif
 
-/**
- * MOZ_MUST_USE tells the compiler to emit a warning if a function's
- * return value is not used by the caller.
- *
- * Place this attribute at the very beginning of a function declaration. For
- * example, write
- *
- *   MOZ_MUST_USE int foo();
- * or
- *   MOZ_MUST_USE int foo() { return 42; }
- *
- * MOZ_MUST_USE is most appropriate for functions where the return value is
- * some kind of success/failure indicator -- often |nsresult|, |bool| or |int|
- * -- because these functions are most commonly the ones that have missing
- * checks. There are three cases of note.
- *
- * - Fallible functions whose return values should always be checked. For
- *   example, a function that opens a file should always be checked because any
- *   subsequent operations on the file will fail if opening it fails. Such
- *   functions should be given a MOZ_MUST_USE annotation.
- *
- * - Fallible functions whose return value need not always be checked. For
- *   example, a function that closes a file might not be checked because it's
- *   common that no further operations would be performed on the file. Such
- *   functions do not need a MOZ_MUST_USE annotation.
- *
- * - Infallible functions, i.e. ones that always return a value indicating
- *   success. These do not need a MOZ_MUST_USE annotation. Ideally, they would
- *   be converted to not return a success/failure indicator, though sometimes
- *   interface constraints prevent this.
- */
-#if defined(__GNUC__) || defined(__clang__)
-#  define MOZ_MUST_USE __attribute__((warn_unused_result))
-#else
-#  define MOZ_MUST_USE
-#endif
-
 /**
  * MOZ_MAYBE_UNUSED suppresses compiler warnings about functions that are
  * never called (in this build configuration, at least).