How to remove DC offset of the input signal without using an opAmp?
Careful - my answer is only valid for low duty cycle signals as shown above

You can use a simple RC high pass filter in front of R3 as your comparator input resistance is quite high.

DC offset is basically Voltage with 0frequency so, the high pass will filter it out!

Something like 47n / 220k will do the job (depending on other design considerations the values may vary!