cmake: Remove support for building static binaries

Building and running static binaries has become increasingly
difficult on modern systems, in particular when involving
complex libraries like Qt due to their reliance on plugins and
other dynamically loaded resources. It is also a security risk,
as upgrades to system libraries won't apply to the static binary.

Having a static quasselcore binary was quite useful when it was
hard to get Qt on an X11-less box, but these days all major distros
support modular Qt, and can provide Quasselcore packages that
don't require graphical dependencies. Consequently, we no longer
intend to offer static binaries.

Thus, remove the corresponding hacks from the build system.