Xcode: Drop hard-coded default warning flags like -Wmost

The -Wmost flag was added by commit 97a51d21, along with a couple others, as part of very early work on the Xcode generator. I suspect that the flags were part of Xcode's project creation wizard. However, they are not appropriate for CMake-based projects because we already have our own mechanisms for specifying flags desired by the project or user. Simply drop the flags.

Fixes: #13516 (closed)