build: remove superfluous options
build: remove superfluous options

These options were already being enabled through the use of `-Wall` or
`-Wextra`.  This shrinks the command line and reduces build file
generation times.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch