Revert "Add C11 standard atomic support (#1645)"

src/rp2_common/CMakeLists.txt

src/rp2_common/pico_atomic/CMakeLists.txt

src/rp2_common/pico_atomic/include/stdatomic.h

src/rp2_common/pico_atomic/pico_atomic.c

src/rp2_common/pico_runtime/CMakeLists.txt