doc: remove SoftDevice Controller changelog entry

Removed the LM20 entry.
Documentation shouldn't mention the board target yet

Signed-off-by: Anna Wojdylo <[email protected]>

Author: annwoj

softdevice_controller/CHANGELOG.rst

@@ -16,7 +16,6 @@ Added
 =====
 
 * Central-only and Peripheral-only library variants for the nRF54L Series devices. (DRGN-25081)
-* Initial prototype support for the nRF54LM20 device (DRGN-24919).
 * Support for generating paEventCounter reports using the VS Enable Periodic Advertising Event Counter Reports command (DRGN-25444).
 
 Bug fixes