[NaCl SDK] Remove redundant output from SDK build system.

The 'rm' commands to remove libraries before creating
them was outputing warnings when the library did not
exist.  This change adds a -f so the warning is not
printed.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/54843007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232251 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232251