As detailed in DEPRECATE.md, the polarssl support is now removed after
having been disabled for 6 months and nobody has missed it.
The threadlock files used by mbedtls are renamed to an 'mbedtls' prefix
instead of the former 'polarssl' and the common functions that
previously were shared between mbedtls and polarssl and contained the
name 'polarssl' have now all been renamed to instead say 'mbedtls'.
Closes#4825
