Remove old 'remove buffer' KeyPress code

Since commit 9f33f6e4 a 'hide current buffer' shortcut is available.
This is the global application shortcut like Sput referenced in
http://bugs.quassel-irc.org/issues/668

Keeping a hard-coded KeyPress event doing the same seems unnecessary.
Users have also expressed dislike in the use of 'Backspace' for this
buffer removal. Unintentional buffer removals are often for some.

This commit removes the 'KeyPress' event and cleans up the
'hideCurrentBuffer' code, removing commented (un-needed) code.

Reference: http://bugs.quassel-irc.org/issues/1403
Fixes #1403. Closes GH-299.