Reduce the size of the code needed to check for Thief Lockpicking

[markyys]

I initially wanted to reduce this to make room for some additional code I plan to add shortly, but it appears not to be needed since it looks like there is plenty of space should it be needed.

In the end I did this more as an exercise in understanding the codebase a little bit, doing a little practical work in 6502 (which I am not very familiar with), and as a way to figure out how to develop and test this sort of chage.

I tested this with no Thief in the party, with a Thief of the requisite level in each slot, and with a Thief of too low a level in at least one slot. I did not test with a Ninja, but I can't imagine that it would behave any differently.