[SPEC] fix for error while uninstalling webapi-plugins on emulator

[SPEC] fix for error while uninstalling webapi-plugins on emulator

On emulator there are two packages webapi-plugins-<profile>-extension-emulator and
webapi-plugins-profile_<profile>. Uninstalling one of them causes error
rm: cannot remove `/usr/lib/tizen-extensions-crosswalk/libtizen*.so': No such file or directory
rm: cannot remove `/usr/lib/tizen-extensions-crosswalk/plugins.json': No such file or directory
That's why it is needed to be check if file exists before remove

Change-Id: I25cf8d854a019637bcd05a3c686d86f4020a1cc2
Signed-off-by: Lukasz Bardeli <l.bardeli@samsung.com>