Revert "subsys/console: Yield on char availability."

This reverts commit 4e2eaec2.

It's invalid to call k_yield from ISR. In fact, it'll trigger an
__ASSERT.

Change-Id: Icc7b81c07c2e7df63fe7d5029fac446ac6fe508b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>