Remove PlainTextPre preference, in favor of PlainTextMono

PlainTextPre was introduced in bb4704f; PlainTextMono was added in
084a6d3 and 3cac06e, but at the time PlainTextPre was left as an option
because it was better-supported under IE6.  As RT 4.2 removes support
for IE6, remove the PlainTextPre option in favor of PlainTextMono, as it
preserves white-space but still allows wrapping of long lines.
Merge branch '4.2/remove-plain-text-pre'

Conflicts:
	etc/RT_Config.pm.in
	share/html/Elements/ShowMessageStanza