remove duplicate LIBOQS_BRANCH option in config doc (open-quantum-saf…

…e#274)

Signed-off-by: Paul S. Schweigert <paul@paulschweigert.com>

There were two entries for LIBOQS_BRANCH in the CONFIGURE doc. This
change drops the second one and makes a slight tweak to the wording of
the first.

Signed-off-by: Felipe Ventura <felipe.ventura@entrust.com>

CONFIGURE.md

@@ -110,8 +110,8 @@ code deficiencies related to providers in such old OpenSSL branches.
 
 This defines the branch of `liboqs` against which `oqs-provider` is built.
 This can be used, for example, to facilitate a release of `oqsprovider`
-to track an old/stable `liboqs` release.
-Default is "main" (most current code).
+to track an old/stable `liboqs` release. If this variable is not set, the
+"main" branch is built.
 
 ### liboqs_DIR
 
@@ -120,11 +120,6 @@ used from the directory specified in this variable: Both `include`
 and `lib` directories must be present in that location.
 By not setting this variable, `liboqs` is build from source.
 
-### LIBOQS_BRANCH
-
-If set, this environment variable designates the `liboqs` branch to
-be built. If this variable is not set, the "main" branch is built.
-
 ### MAKE_PARAMS
 
 This environment variable permits passing parameters to the `make`