chore(README): Remove outdated instructions for OpenSSL

Since 2022 (OpenSSL 3 support in Deimos), the way to select the OpenSSL version
has changed, and those instructions are no longer accurate. The code for it is
now in vibe-stream, which is a separate package.
Instead of updating those instructions, I think it is better to remove them,
as at the time they were added, it was a common problem to have to select
the OpenSSL version. However, with the auto-detect script now in Deimos/OpenSSL,
we are no longer seeing issues about this.

Author: Geod24

README.md

@@ -168,36 +168,3 @@ Install the DMD compiler and vibe.d's dependencies using portupgrade or a simila
     sudo portupgrade -PN devel/pkgconf
 
 Optionally, run `./setup-freebsd.sh` to create a user/group pair for privilege lowering.
-
-
-Switching between OpenSSL versions
-----------------------------------
-
-By default, vibe.d is built against OpenSSL 1.1.x. On systems that use the older
-1.0.x branch, this can be overridden on the DUB command line using
-`--override-config vibe-d:tls/openssl-1.0`. Alternatively, the same can be done
-using a sub configuration directive in the package recipe:
-
-SDL syntax:
-```
-dependency "vibe-stream:tls" version="~>1.0"
-subConfiguration "vibe-stream:tls" "openssl-1.0"
-```
-
-JSON syntax:
-```
-{
-    ...
-    "dependencies": {
-        ...
-        "vibe-stream:tls": "~>1.0"
-    },
-    "subConfigurations": {
-        ...
-        "vibe-stream:tls": "openssl-1.0"
-    }
-}
-```
-
-Finally, there is a "botan" configuration for using the D port of the Botan
-library.