package/libwebsockets: remove dependency on mmu

Six years ago, commit b6b5bb51 added the MMU
dependency for the libwebsockets package. However, according to the git
history of the CMakeLists.txt file, libwebsockets has supported the vfork
function for at least the last five years.

After testing with the qemu_arm_versatile_nommu_defconfig and the
br-arm-cortex-m4-full.config file, no errors occurred when building
libwebsockets without MMU support.

Remove the dependency; it is no longer necessary. Update the reverse
dependencies as needed.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>