drivers: modem: remove deprecated quectel-bg9x references

By generalizing use of BG9X devices in modem cellular, it is necessary
to cleanup old files and references to quectel_bg9x.c.

Signed-off-by: Guilherme Costa <guilhermecosta@stratioautomotive.com>

Author: guildeol-stra

drivers/modem/Kconfig

@@ -188,7 +188,6 @@ config MODEM_CELL_INFO
 	  Query for numerical operator id, location area code and cell id.
 
 source "drivers/modem/Kconfig.ublox-sara-r4"
-source "drivers/modem/Kconfig.quectel-bg9x"
 source "drivers/modem/Kconfig.wncm14a2a"
 source "drivers/modem/Kconfig.cellular"
 source "drivers/modem/Kconfig.at_shell"