gdbserver: regcache: Update comment in supply_regblock

Since commit 84da4a1ea0 ("gdbserver: refactor the definition and uses of
supply_regblock") there is no case where supply_regblock is passed a
nullptr for the BUF argument, and there is even a gdb_assert to make
sure of it.

Therefore remove that part of the documentation comment.

gdbserver/regcache.cc

@@ -353,8 +353,7 @@ supply_register_by_name_zeroed (struct regcache *regcache,
 #endif
 
 /* Supply the whole register set whose contents are stored in BUF, to
-   REGCACHE.  If BUF is NULL, all the registers' values are recorded
-   as unavailable.  */
+   REGCACHE.  */
 
 void
 supply_regblock (struct regcache *regcache, const void *buf)