Revert "kernel: init: activate FPU for main thread"

Activating K_FP_REGS flags introduces stack memory
overhead for the main thread in Cortex-M architecture.
Several ARM platforms experience main thread stack
overflows when building with FPU_SHARING=y.
Enabling FPU sharing in main thread should not be
the default configuration. Users are welcome to
enable FP sharing on the main thread in the
application code, in main().

This reverts commit 8453a73e.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>