configure.ac: Remove duplicated CMD_DESCR_ROTATE definition

CMD_DESCR_ROTATE is defined twice by configure.  This breaks configuring
with "-Werror -Wall", as some test programs don't compile due to:

  configure:16990: gcc -o conftest -Wall -Werror   conftest.c  >&5
  conftest.c:163: error: "CONFIG_CMD_DESCR_ROTATE" redefined [-Werror]
   #define CONFIG_CMD_DESCR_ROTATE "Archive a tracing session’s current trace chunk"

  conftest.c:154: note: this is the location of the previous definition
   #define CONFIG_CMD_DESCR_ROTATE "Archive a tracing session's current trace chunk"

  cc1: all warnings being treated as errors

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>