zsh completion: remove computed default values from help texts

The default encoding and encoding mode are computed at runtime and can
vary from system to system. The two remaining default values in the
completion help texts (number of directories to strip on import and
default bundle name) are fixed in the source code.

contrib/zsh_completion

@@ -360,8 +360,8 b' typeset -A _hg_cmd_globals'
     '(--help -h)'{-h,--help}'[display help and exit]'
     '--debug[debug mode]'
     '--debugger[start debugger]'
-    '--encoding[set the charset encoding (default: UTF8)]'
-    '--encodingmode[set the charset encoding mode (default: strict)]'
+    '--encoding[set the charset encoding]'
+    '--encodingmode[set the charset encoding mode]'
     '--lsprof[print improved command execution profile]'
     '--traceback[print traceback on exception]'
     '--time[time how long the command takes]'