Revert "Sema: Infer '@_usableFromInline' imports by looking at references from inlinable functions (#16326)"

This reverts commit ee6e190e09. It's not
sufficient to solve the problem, and the choices were to do something
more complicated, or just take a simple brute force approach. We're
going with the latter.

lib/AST/Module.cpp

@@ -1287,14 +1287,6 @@ bool SourceFile::hasTestableImport(const swift::ModuleDecl *module) const {
   });
 }
 
-void SourceFile::markUsableFromInlineImport(const ModuleDecl *module) {
-  for (auto &Import : Imports) {
-    if (Import.first.second == module) {
-      Import.second |= ImportFlags::UsableFromInline;
-    }
-  }
-}
-
 void SourceFile::clearLookupCache() {
   if (!Cache)
     return;