arm/imxrt: Remove extra lock from imxrt_edma

An extra `lock` appears in imxrt_edma due to 2 conflicting PRs:
- apache#15352
- apache#15353

This PR removes the extra `lock` in imxrt_edma.

Author: lupyuen

arch/arm/src/imxrt/imxrt_edma.c

@@ -145,7 +145,6 @@ struct imxrt_edma_s
   /* This array describes each DMA channel */
 
   struct imxrt_dmach_s dmach[IMXRT_EDMA_NCHANNELS];
-  spinlock_t lock;                /* Spinlock */
 };
 
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