Deprecate signature related feature flags

[Zangetsu101]

Signature fields are not part of form configurations so the feature flags that we have in application-config related to signature fields are deprecated.

• Remove INFORMANT_SIGNATURE & INFORMANT_SIGNATURE_REQUIRED from core & countryconfig
• Write CHANGELOG entry regarding their depreciation

[Zangetsu101]

@marcio-opencrvs I am adding this to the 1.6.3 milestone as this is something related to missing documentation for 1.6.0 I guess we are not doing 1.6.3 so 1.7.0 it is then

[marcio-opencrvs]

@Zangetsu101 that's a good one. This should be marked 1.7.0 if it came up during regression testing AND needs fixing before the full release. Otherwise, this could be moved to a next small release (1.6.3... or 1.7.1, depending on the timing of the ticketd completion...)