Platforms.txt: Remove x86_64/unknown/linux-gnu, tcc 0.9.25

This combination had been successfully tested with ngIRCd 21, but after
further investigation didn't build correctly: it seems as if tcc fails
to correctly link external libraries (e. g. ngipaddr).

Astonishingly the test suite passed nevertheless, with garbled output,
but without the daemon crashing!?

When using tcc with ngIRCd 23 (and current master), the test suite fails
completely because the daemon crashes ... (which actually is good!)