revset: remove unused 'only' from methods table

The infix 'only' operator is mapped to 'only()' function by optimize(), so
it won't be looked up as a method. The test shows it.

mercurial/revset.py

@@ -2094,7 +2094,6 b' methods = {'
     "ancestor": ancestorspec,
     "parent": parentspec,
     "parentpost": p1,
-    "only": only,
 }
 
 def optimize(x, small):

tests/test-revset.t

@@ -615,6 +615,24 b" Test opreand of '%' is optimized recursi"
   <baseset+ [8, 9]>
   8
   9
+  $ try --optimize '(9)%(5)'
+  (only
+    (group
+      ('symbol', '9'))
+    (group
+      ('symbol', '5')))
+  * optimized:
+  (func
+    ('symbol', 'only')
+    (list
+      ('symbol', '9')
+      ('symbol', '5')))
+  * set:
+  <baseset+ [8, 9, 2, 4]>
+  2
+  4
+  8
+  9
 
 Test the order of operations