Bluetooth: logging: don't imply LOG_FUNC_NAME_PREFIX_*

By default there is only CONFIG_LOG_FUNC_NAME_PREFIX_DBG=y enabled. When
both Bluetooth (CONFIG_BT=y) and logging (CONFIG_LOG=y) subsystems are
enabled, then other CONFIG_LOG_FUNC_NAME_PREFIX_{INF,WRN,ERR}=y options are
pulled in as well using an 'imply' Kconfig command indirectly from
`CONFIG_BT_LOG`. This behavior was introduced recently as part of
commit af01a0f3 ("Bluetooth: Logging: Move all logging symbols
together") with no explicit reason provided.

Pulling in LOG_FUNC_NAME_PREFIX_* options automatically with (CONFIG_BT=y
&& CONFIG_LOG=y) blows up flash usage. As an example of downstream project
(nRF52840-based, with Bluetooth and WiFi connectivity), it increases flash
usage from 473668 bytes to 487856 bytes. This seems "only" 3% difference,
but this is actually a lot when there is no good reason why this happens.
Downstream users quite often compare flash sizes of subsequent Zephyr
releases and this 3% footprint increase might be a blocker at some point.
Additionally, it is not trivial to find the root cause of footprint
increase for most (non-expert) users.

The reason why there is so much footprint overhead because of
CONFIG_LOG_FUNC_NAME_PREFIX_*=y is because each function in the
codebase (be it Zephyr or downstream application) that contains logging
macros (LOG_{DBG,INF,WRN,ERR}()) is bloated because the function name has
to be included in the output image.

Remove 'imply LOG_FUNC_NAME_PREFIX_*' commands from 'menuconfig BT_LOG'
option, so that flash usage does not increase too much. Those logging
options are not enabled by other subsystems, so Bluetooth should not be an
exception here.

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