graphics/libosmesa: Remove expat from LIB_DEPENDS

It's not needed for this mesa component.

Differential Revision: https://reviews.freebsd.org/D44828
PR: 278324
Reported by: autodep
Reviewed by: bapt
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit bed3b0ad509eca940c9a368658a69c9e6210cdec)