kconfig: Remove remaining 'option env' bounce symbols

These are no longer needed since commit 4dc9e5b2 ("kconfig: Get rid
of 'option env' bounce symbols"). The C tools are likely to get rid of
them soon too.

The APPLICATION_BASE symbol (option env="PROJECT_BASE") triggered a
compatibility warning, because 'option env' symbols now need to have the
same name as the environment variables they reference to be compatible
with Kconfiglib. APPLICATION_BASE is unused, so just remove it.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>