Bug #705123: qofbookmerge.c: bad if statement

Remove qofbookmerge, which isn't used anywhere and which has
already been removed from trunk.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@23107 57a11ea4-9604-0410-9ed3-97b8803252fd