bitcoin: poller: fix spend expiration from mempool, again

[darosior]

Always mark a spend dropped from mempool as expired if it, or a conflict spending this coin, wasn't mined. The previous logic was confused: it would only do so if a conflict was detected, but it's not the only reason for a tx to be dropped from the mempool.

[jp1ac4]

ACK 758a4d6.

I tested with the following scenario:

A ---\	    / D
     T_1 --- 
B ---/	    \ E ----\
		     T_2
C ------------------/

Transaction T_2 spends an output from T_1. I then replaced T_1 using RBF, which also removed T_2. The spend info from coin C was cleared as expected.