accessibility/at-spi2-core: drop unused X11 deps after 951c1f5d1c73

Disabling X11 only affects the following libraries. -Wl,--as-needed
removes the rest via https://github.com/mesonbuild/meson/commit/58ca96994f5a

$ poudriere testport -j 131amd64 -z nox11 accessibility/at-spi2-core
[...]
=>> Checking shared library dependencies

• 0x0000000000000001 NEEDED Shared library: [libX11.so.6]
• 0x0000000000000001 NEEDED Shared library: [libXi.so.6]
• 0x0000000000000001 NEEDED Shared library: [libXtst.so.6]

[...]