gnu: pavucontrol: Update to 3.0.

* gnu/packages/pulseaudio.scm (pavucontrol): Update to 3.0.
* gnu/packages/patches/pavucontrol-sigsegv.patch: Remove.
* gnu-system.am (dist_patch_DATA): Remove it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>