Revert "flatpak: Disable obs-browser build"

This reverts commit c370692773.

The PR that introduced this commit was opened before the browser plugin
was enabled, and when the browser was effectively enabled, this change
was already in place and didn't conflict, leaving us with both a
-DBUILD_BROWSER=ON *and* -DBUILD_BROWSER=OFF in place.

CI/flatpak/com.obsproject.Studio.json

@@ -259,8 +259,7 @@
         "-DUSE_XDG=ON",
         "-DDISABLE_ALSA=ON",
         "-DENABLE_PULSEAUDIO=ON",
-        "-DWITH_RTMPS=ON",
-        "-DBUILD_BROWSER=OFF"
+        "-DWITH_RTMPS=ON"
       ],
       "sources": [
         {