Linear Pool: consider removing minimumBPT override

[EndymionJkb]

Most pools mint an initial (very small) amount of BPT to the zero address on initialization, so that the pool can never be completely drained. Most importantly, this prevents _onInitializePool from being called more than once. (And also helps avoid math issues at very low liquidity.)

Linear Pools override and set this to 0, since the preminting also avoids any such issue (as it would with any composable pool). This results in a slight increase in complexity (e.g., we have to override a function, and we also get an event from minting 0 tokens), so we could remove this, and let it mint the premint amount + epsilon, same as other pools.