Undeprecate DH_get_length() and DH_set_length() functions

Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11225)

doc/man3/DH_get0_pqg.pod

@@ -96,8 +96,7 @@ object, or NULL if no such ENGINE has been set. This function is deprecated.
 The DH_get_length() and DH_set_length() functions get and set the optional
 length parameter associated with this DH object. If the length is nonzero then
 it is used, otherwise it is ignored. The I<length> parameter indicates the
-length of the secret exponent (private key) in bits. These functions are
-deprecated.
+length of the secret exponent (private key) in bits.
 
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