Remove already-deprecated wallet commands and options

[andreibancioiu]

Some commands and command arguments were deprecated in mxpy 6. Now (for mxpy 7), we remove them.

Removed:

• command mxpy wallet derive. One should now use mxpy wallet convert, instead.
• arguments --pem, --json, --output-path of command mxpy wallet new. One should now use --format=... and --outfile parameters, instead

Replacement examples:

• mxpy wallet derive alice.pem --mnemonic is replaced by mxpy wallet convert --in-format=raw-mnemonic --out-format=pem --outfile=alice.pem
• mxpy wallet new --pem --output-path=alice.pem is replaced by mxpy wallet new --format=pem --outfile=alice.pem

See:

• https://github.com/multiversx/mx-sdk-py-cli/blob/main/CLI.md#walletnew
• https://github.com/multiversx/mx-sdk-py-cli/blob/main/CLI.md#walletconvert