remove BORG_LIBB2_PREFIX (not used any more)

the code now uses hashlib.blake2b from python stdlib.

docs/usage/general/environment.rst.inc

@@ -156,9 +156,6 @@ Building:
     BORG_LIBLZ4_PREFIX
         Adds given prefix directory to the default locations. If a 'include/lz4.h' is found Borg
         will be linked against the system liblz4 instead of a bundled implementation. (setup.py)
-    BORG_LIBB2_PREFIX
-        Adds given prefix directory to the default locations. If a 'include/blake2.h' is found Borg
-        will be linked against the system libb2 instead of a bundled implementation. (setup.py)
     BORG_LIBZSTD_PREFIX
         Adds given prefix directory to the default locations. If a 'include/zstd.h' is found Borg
         will be linked against the system libzstd instead of a bundled implementation. (setup.py)