www/piwigo: Fix build error with php70

This is done by removing the obsoleted mysql module from USE_PHP,
especially since its replacement mysqli is already in it.

PR: 217318
Submitted by: tz
Reported by: Marko Cupać <marko.cupac@mimar.rs>
Reviewed by: Marko Cupać <marko.cupac@mimar.rs>
Approved by: Ralf van der Enden (maintainer)