tests: kernel: timer_api: remove extra adjustment to expected result

The converted target value for remaining ticks was increased by one to
match original code, which used a one-sided test.  The current test is
two-sided, so that increment is already present in the allowed 1 tick
error for boards with no slew, and incorporating it into the absolute
error can cause the test to fail.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>