cmake: remove spurious IS_ABSOLUTE logic in zephyr_sources()

target_sources() documentation states:

  Relative source file paths are interpreted as being relative to the
  current source directory (i.e. CMAKE_CURRENT_SOURCE_DIR).

Remove spurious code duplicating cmake's behaviour. It proved to be a
time-consuming red herring while debugging some path-related issue and
"less is more".

Signed-off-by: Marc Herbert <marc.herbert@intel.com>