graphics/libdrm: Remove @${PY_FLAVOR}

While it works fine when building directly the port poudriere complains :
[00:00:06] Warning: (graphics/libdrm): Error: graphics/libdrm has dependency on textproc/py-docutils with invalid empty FLAVOR; Please contact maintainer of the port to fix this.

portlint suggested this but this seems wrong.

Fixes: 67c3a731efe7 ("graphics/libdrm: Always enable manpages")
Sponsored by: Beckhoff Automation GmbH & Co. KG