print/cups{,15}: Don't make ${CUPS_CONFDIR} with INSTALLATION_DIRS.

${CUPS_CONFDIR} (${PKG_SYSCONFDIR}) may be located outside of
${PREFIX}, which causes a installation failure due to the checks
added in mk/install/install.mk.

Remove ${CUPS_CONFDIR} from INSTALLATION_DIRS since it is created
by the package install script at PRE-INSTALL time.

Fixes pkg/52481 reported by Hauke Fath.

(jlam)