extdiff: drop the command alias without options example in the help text

In the dropped example, the extension would look for 'vdiff.diffargs' in the
configuration, and not finding it, would run kdiff3 without the configured
options.  That's not obvious to a new user who sees a kdiff3 configuration in
the prepackaged mergetools.rc file, and sees that kdiff3 still runs.  While it
is conceivable that the user wants a kdiff3 command that runs without the
preconfigured options, it is more likely what they want is this, which uses the
canned options:

    [alias]
    vdiff = kdiff3

    [extdiff]
    kdiff3 =

We could mention alias here, but that seems like it belongs elswhere.

hgext/extdiff.py

@@ -23,9 +23,6 b' you do not need to type :hg:`extdiff -p '
   #cmd.cdiff = gdiff
   #opts.cdiff = -Nprc5
 
-  # add new command called vdiff, runs kdiff3
-  vdiff = kdiff3
-
   # add new command called meld, runs meld (no need to name twice).  If
   # the meld executable is not available, the meld tool in [merge-tools]
   # will be used, if available

tests/test-extension.t

@@ -424,9 +424,6 b' Extension module help vs command help:'
     #cmd.cdiff = gdiff
     #opts.cdiff = -Nprc5
   
-    # add new command called vdiff, runs kdiff3
-    vdiff = kdiff3
-  
     # add new command called meld, runs meld (no need to name twice).  If
     # the meld executable is not available, the meld tool in [merge-tools]
     # will be used, if available