crimson/osd: remove redundant comments

crimson/osd: remove redundant comments

Remove comments that merely restate what the code already says clearly:
- SeastarOption field comments (option_name, config_key, value_type)
- "Define a list of Seastar options" above seastar_options
- "Function to get the option value as a string" above get_option_value
- "stop()s registered using defer() are called here" in main()

Also fix the trailing space before the semicolon in the value_type
field declaration.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>