Remove WatchOS support from wangle

Summary:
This should not be used anymore, so remove to generate fewer targets.

See also https://fb.workplace.com/groups/iosdevxadhoc/posts/1494056874808920

Reviewed By: rmaz

Differential Revision: D62029477

fbshipit-source-id: 427b84a92c8c882a4b8c77f89605ae22a805ecf2

wangle/defs.bzl

@@ -9,7 +9,6 @@ load(
     "FBCODE",
     "IOS",
     "MACOSX",
-    "WATCHOS",
     "WINDOWS",
 )
 
@@ -47,7 +46,7 @@ WINDOWS_CLANG_CXX_FLAGS = [
     "-DBOOST_HAS_THREADS",
 ]
 
-DEFAULT_APPLE_SDKS = (IOS, MACOSX, WATCHOS)
+DEFAULT_APPLE_SDKS = (IOS, MACOSX)
 
 DEFAULT_PLATFORMS = (ANDROID, APPLE, CXX, FBCODE, WINDOWS)