build: Remove tap/configure.in

We already have a duplicate configure.ac, which is what the file should
be named these days.

Aside from saving us a few bytes, this also silences the following
warnings:

aclocal: warning: autoconf input should be named 'configure.ac', not
'configure.in'
automake: warning: autoconf input should be named 'configure.ac', not
'configure.in'
automake: warning: autoconf input should be named 'configure.ac', not
'configure.in'

which is pretty nice in and of itself.

Signed-off-by: Anton Lofgren <alofgren@op5.com>