SimulateTrade is incorrectly included in "expensive RPCs"

[hdevalence]

Describe the bug

pd contains a flag, --enable-expensive-rpc, that enables RPC endpoints believed to be more expensive and therefore unsuitable to expose by default.

Currently, this includes the SimulateTrade RPC, which turns out to not be expensive, as long as the node is running on hardware with a reasonable SSD.

The SimulateTrade RPC should be moved out of the "expensive RPC" category, so it is enabled by default.

This may mean the category is empty. That's fine. The flag can be a no-op, preserving the same semantics -- in case expensive RPC is ever added in the future. This avoids changing deployment setups.