[MC] Restore default section classification to SectionKind::getReadOnly()

D133456 changed the default `Kind = SectionKind::getReadOnly();` to
`SectionKind::getText()` to work around a longstanding issue in producing
R_RISCV_{ADD,SUB}* relocations.

D155357 properly fixed the issue and the workaround is no longer needed.
getReadonly() actually matches BFD better as BFD doesn't set SEC_CODE to
a not-well-known section.

To the best of my knowledge, this change is un-testable after D155357
removed a `getKind().isText()` use.