drivers: sensor: lis2mdl: kconfig: Remove unused fixed ODR symbols

After commit 44f373e8 ("driver/sensor: lis2mdl: make use of STdC
definitions"), the code only looks at LIS2MDL_MAG_ODR_RUNTIME, and not
at the LIS2MDL_MAG_ODR_<frequency> symbols.

LIS2MDL_MAG_ODR_RUNTIME is now a yes/no thing in practice, so remove the
choice and turn it into a regular bool symbol.

Found with a script.

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