Revert "config: enable BIGMEM and set VRAM size to 12MB"

This reverts commit 3225d65020294c050f3074658d630eb3fb9341c7.
Revert "HUGEMEM: rename to BIGMEM and add 8 MB to HEAP_TILER again"

This reverts commit 94a2f590b06011f115a73151bf66ec68a3d5cc22.
Revert "Tuna: add option to reclaim ~73 MB of reserved RAM (HUGEMEM)"

This reverts commit 2ad10294bda56ff114b00415dca873b935bc410d.