gnu: icecat: Include generally-useful fixes in the source tarball.

gnu: icecat: Include generally-useful fixes in the source tarball.

This commit moves some important fixes into a patch applied to the upstream
gnuzilla git repository, whereas previously they were applied in such a way
that only benefitted Guix users.

* gnu/packages/patches/icecat-default-search-ddg.patch,
gnu/packages/patches/icecat-disable-sync.patch: Delete files.
* gnu/packages/patches/icecat-gnuzilla-fixes.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adapt accordingly.
* gnu/packages/gnuzilla.scm (icecat-source): Apply the new patch to the
gnuzilla checkout.
(icecat)[native-inputs]: Remove deleted patches.
[arguments]: In the 'wrap-program' phase, remove MOZ_LEGACY_PROFILES=1
from the wrapper.