Remove gt/gT bindings

Those were added in #443, inspired by luakit.

However, all other bindings follow dwb's defaults, and dwb uses `gt` for
showing buffers. To be consistent, let's rebind gt to show :buffer.

qutebrowser/config/configdata.py

@@ -1397,8 +1397,8 @@ KEY_DATA = collections.OrderedDict([
         ('tab-move', ['gm']),
         ('tab-move -', ['gl']),
         ('tab-move +', ['gr']),
-        ('tab-focus', ['J', 'gt']),
-        ('tab-prev', ['K', 'gT']),
+        ('tab-focus', ['J']),
+        ('tab-prev', ['K']),
         ('tab-clone', ['gC']),
         ('reload', ['r']),
         ('reload -f', ['R']),