tests: settings: remove unused settings_enc.c

This file was just built, but none of its functions were executed since the
commit 84a88f10 ("tests: subsys: settings: stream encoding tests"),
which introduced it.

At this point it makes little sense to make it run/working, mainly because
of:
 * "line" encoding, which this module intends to test, behaves differently
   based on Kconfig options like `bool SETTINGS_ENCODE_LEN`; this is set
   differently based on (FCB/File) backend used, so it is hard to make this
   "generic" tests work, because it is shared by all storage backends tests
 * base64 encoding is expected in this test module, which was dropped 2
   years ago in [1]
 * settings subsystem has changed a lot during last 4 years, so those tests
   might no longer be valid with current implementation.

[1] commit 55be783c ("settings: Remove deprecated SETTINGS_USE_BASE64
  support")

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>