hash: remove redundant secp256k1_sha256_initialize in tagged hash midstate functions

[w0xlt]

Each tagged hash midstate function (e.g., secp256k1_schnorrsig_sha256_tagged) calls secp256k1_sha256_initialize before immediately overwriting every field it sets: s[0] through s[7] and bytes. The buf[64] member does not need initialization either, because bytes is set to 64, which means the buffer position (bytes & 0x3F) (= bytes % 64) is 0, so buf is always written before being read.

Remove the 11 redundant secp256k1_sha256_initialize calls across the schnorrsig, ellswift, and musig modules.