[M2351] Remove special handling for dummy interrupt in lp_ticker
[M2351] Remove special handling for dummy interrupt in lp_ticker

It is because dummy interrupt is very rare or pending time caused by it
is very short.

kjbracey-arm/mbed,betzw/mbed-os,betzw/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os