Use FK to Skills in HospitalDoctor instead of DOCTOR_TYPES

[sainak]

Since this is now same as the records in Skills table, we should use foreign key relation to the skill table to track the count.

Originally posted by @rithviknishad in https://github.com/coronasafe/care/pull/2102#pullrequestreview-2054905724

[sainak]

@vigneshhari @gigincg thoughts on dropping HospitalDoctors table?

[gigincg]

@sainak There can be more Skills than Doctor Specializations.