ath11k: Remove unused variable in ath11k_dp_rx_mon_merg_msdus()

Coverity complains that a constant variable guards dead code. In fact,
mpdu_buf is set NULL and never updated.

4834err_merge_fail:
    	null: At condition mpdu_buf, the value of mpdu_buf must be NULL.
    	dead_error_condition: The condition mpdu_buf cannot be true.
CID 92162 (#1 of 1): 'Constant' variable guards dead code (DEADCODE)
dead_error_line: Execution cannot reach the expression decap_format !=
  DP_RX_DECAP_TYPE_RAW inside this statement: if (mpdu_buf && decap_forma....
Local variable mpdu_buf is assigned only once, to a constant value, making it
  effectively constant throughout its scope. If this is not the intent, examine
  the logic to see if there is a missing assignment that would make mpdu_buf not
  remain constant.
4835        if (mpdu_buf && decap_format != DP_RX_DECAP_TYPE_RAW) {

Fix this by removing mpdu_buf and unreachable code.

Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: ath11k@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210927150743.19816-1-tim.gardner@canonical.com