ci: remove additional linux-gcc-14 when ubuntu-latest becomes ubuntu 24.04

[diegomrsantos]

An additional target using Ubuntu 24.04 was added to make the GCC 14 the compiler and test the code against it. Once ubuntu-latest becomes ubuntu 24.04 and the GCC 14 compiler becomes the default one, this must be removed.

The motivation for this addition was to test on CI the fixes for https://gcc.gnu.org/gcc-14/porting_to.html#incompatible-pointer-types