polarssl: removed

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

docs/DEPRECATE.md

@@ -5,32 +5,8 @@ email the curl-library mailing list as soon as possible and explain to us why
 this is a problem for you and how your use case can't be satisfied properly
 using a work around.
 
-## PolarSSL
+## Past removals
 
-The polarssl TLS library has not had an update in over three years. The last
-release was done on [January 7
-2016](https://tls.mbed.org/tech-updates/releases). This library has been
-superseded by the mbedTLS library, which is the current incarnation of
-PolarSSL. curl has supported mbedTLS since 2015.
-
-It seems unlikely that this library is a good choice for users to get proper
-TLS security and support today and at the same time there are plenty of good
-and updated alternatives.
-
-I consider it likely that the existing users of curl + polarssl out there are
-stuck on old curl versions and when they eventually manage to update curl they
-should also be able to update their TLS library.
-
-### State
-
-In the curl 7.65.2 release (July 17, 2019) the ability to build with this TLS
-backend is removed from the configure script. The code remains and can be
-built and used going forward, but it has to be manually enabled in a build (or
-the configure removal reverted).
-
-### Removal
-
-The support for PolarSSL and all code for it will be completely removed from
-the curl code base six months after it ships disabled in configure in a
-release. In the release on or near February 27, 2020. (possibly called curl
-7.70.0).
+ - Pipelining
+ - axTLS
+ - PolarSSL