Do not force multi-threaded mode with wmo

When multi-threaded mode is enabled, the compiler tries to generate one
object file per source file. This makes us hit a known issue where linking
static libraries fails in loading types metadata, raising
`"failed to demangle superclass of MyClass from mangled name '<mangled name>'"`.

Using single thread wmo makes the compiler generate a single object
file, so all symbols are linked.

Ref. https://developer.apple.com/documentation/xcode_release_notes/xcode_10_2_release_notes/swift_5_release_notes_for_xcode_10_2#3136859

Author: Alessandro Patti

swift/internal/api.bzl

@@ -477,15 +477,6 @@ def _compile(
         *   `swiftmodule`: The `.swiftmodule` file that was produced by the compiler.
     """
 
-    # Force threaded mode for WMO builds, using the same number of cores that is
-    # on a Mac Pro for historical reasons.
-    # TODO(b/32571265): Generalize this based on platform and core count when an
-    # API to obtain this is available.
-    if _is_wmo(copts + swift_toolchain.command_line_copts, feature_configuration):
-        # We intentionally don't use `+=` or `extend` here to ensure that a
-        # copy is made instead of extending the original.
-        copts = copts + ["-num-threads", "12"]
-
     compile_reqs = declare_compile_outputs(
         actions = actions,
         copts = copts + swift_toolchain.command_line_copts,