core/api-spec: remove indexes
core/api-spec: remove indexes

Indexes don't currently provide any performance benefit over
regular filters. This commit proposes to remove them from the API
until they are fully implemented.

Closes chain/chainprv#1690
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain