Drop the memory pool implementation

[mbitsnbites]

The memory pool allocator is in conflict with the newlib malloc()/sbrk() allocator.

Consider replacing it with a simple LIFO allocator for VRAM, e.g. using the __vram_free_start symbol provided by the linker scripts, and possibly MMIO(VRAMSIZE). For instance mc1-quake & mc1-doom use a similar (custom) approach already.