graphics/mesa-devel: drop DragonFly cruft
- DPorts hasn't updated since 22.2.b.1392 (2022-08-31)
- DPorts patched to no longer excludes libunwind and anv
- Allow DPorts to pick libelf.so from devel/elfutils
(cherry picked from commit 5b9dfca041d81d77478594609bcb9285a3239070)