configure: drop -flto-partition=one

configure: drop -flto-partition=one

For me, -flto-partition=one takes ~35m to build + test, while the default
(which is 'balanced') takes ~5m.

The reason that -flto-partition=one is slower is because it disables all
of gcc's LTO parallelisation. This can produce better code, at the cost
of (far) more expensive build times. If users want that, they can still
pass it in their *FLAGS, but I don't think it's a suitable default.

This was originally added in ab773096df7aaaf46e8883af5ed4690f4d4499af.

https://bugs.kde.org/show_bug.cgi?id=490651