histedit: remove an unnecessary default value of `b''` for commit message

I don't think `ctx.description()` is ever anything falsy other than
`b''`. I think the comment added in 4c4232e51167 (histedit: extract
common summary code into method, 2016-05-27) is mistaken (I don't see
any code following that pattern before the function was added).

Differential Revision: https://phab.mercurial-scm.org/D12406

hgext/histedit.py

@@ -2319,10 +2319,7 b' def _newhistedit(ui, repo, state, revs, '
 
 
 def _getsummary(ctx):
-    # a common pattern is to extract the summary but default to the empty
-    # string
-    summary = ctx.description() or b''
-    return stringutil.firstline(summary)
+    return stringutil.firstline(ctx.description())
 
 
 def bootstrapcontinue(ui, state, opts):