proftpd: fix do_install failure after libexecdir change

As of OE-Core commit id 406bd38b4232f9f399ef5ffe0b4fac72ed605a23
libexecdir has been changed to ${libdir}/${BPN}. As we are already
deleting ${libdir}/proftpd which is the same as ${libdir}/${BPN} in this
case, introduce a check to see if ${libexecdir} still exists before
attempting to remove it possibly a second time. This will handle if
libexecdir is set to its original value or a custom value in the distro
config.

Based on a patch by Kevin Strasser <kevin.strasser@linux.intel.com>

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>