build,qt: Drop broken check for -fPIC

* m4/qt6.m4: Remove attempt to build a Qt program with -fPIC.

libtool already takes care of adding -fPIC. Moreover, building without
-fPIC succeeded even if Qt was built with -fPIC, i.e. the check didn't
work as intended.

• GnuPG-bug-id: T6696