Remove unused defines from PlatformExportMacros.h
Remove unused defines from PlatformExportMacros.h

WTF_USE_EXPORT_MACROS and WTF_USE_EXPORT_MACROS_FOR_TESTING weren't used in
blink (both were deleted in https://codereview.chromium.org/14065009).

The file can be deleted completely soon.

BUG=none

Review URL: https://codereview.chromium.org/14067004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148126 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs