drivers/hwtracing/stm: Remove "select SRCU"

Now that the SRCU Kconfig option is unconditionally selected, there is
no longer any point in selecting it.  Therefore, remove the "select SRCU"
Kconfig statements.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: <linux-stm32@st-md-mailman.stormreply.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>