Revert "drivers: serial: ns16550: Fix TX IRQ not triggered...

...when FIFO is empty"

This reverts commit 47e43d55.

This is breaking sample.sensor.shell.pytest where characters
are either missing or repeated when printing to the console.

Originally this is just for RISC-V with PLIC interrupt
controller. That was made more general to avoid having arch
specific code in a generic driver. And now it is breaking
on non-RISC-V platforms. Note that the QEMU RISC-V boards
all have PLIC as interrupt controller and are passing sensor
shell pytest without this workaround. So this does not seem
to be an issue with PLIC and NS16550.  So revert the commit
for now.

Fixes #92187

Signed-off-by: Daniel Leung <daniel.leung@intel.com>