firefox: Remove SNAP_NAME=firefox in wrapper

082ed38 introduced it to fix the profile-per-install policy of FF 67. But since
FF 69 (or 68?), there is `MOZ_LEGACY_PROFILES`, which we use since 87e2618.
There is no reason for the `SNAP_NAME=firefox` workaround anymore.

Additionally, the combination of `SNAP_NAME=firefox` with
a large ~/.nix-profile/share in `XDG_DATA_DIRS` triggered
https://bugzilla.mozilla.org/show_bug.cgi?id=1569625 for me, so this really
fixes a bug in my configuration.

The only downside of this approach is that we lose support for running FF 67
(and possibly 68).

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -263,7 +263,6 @@ let
             --suffix PATH ':' "$out${browser.execdir or "/bin"}" \
             --set MOZ_APP_LAUNCHER "${browserName}${nameSuffix}" \
             --set MOZ_SYSTEM_DIR "$out/lib/mozilla" \
-            --set SNAP_NAME "firefox" \
             --set MOZ_LEGACY_PROFILES 1 \
             --set MOZ_ALLOW_DOWNGRADE 1 \
             ${lib.optionalString forceWayland ''