Remove unused variable from AudioBufferManager class #2594

Job	Run time
Build RP2350-RISCV 0	6m 44s
Build RP2350-RISCV 1	7m 30s
Spelling, Style, Boards, Package	1m 10s
Mac (macOS-12)	1s
Build RP2350-RISCV 2	7m 42s
Build 0	7m 25s
Mac (macOS-14)	2m 29s
Build RP2350-RISCV 3	6m 44s
Build 1	7m 42s
Build PlatformIO Examples	4m 1s
Build RP2350-RISCV 4	7m 25s
Build 2	8m 8s
Build RP2350-RISCV 5	7m 52s
Build 3	8m 23s
Build RP2350 0	7m 21s
Build TinyUSB Examples	4m 28s
Build 4	8m 21s
Build RP2350 1	6m 57s
Build 5	8m 18s
Build Every Variant 0	7m 16s
Build RP2350 2	7m 58s
Windows	3m 31s
Build RP2350 3	6m 51s
Build Every Variant 1	7m 7s
Build RP2350 4	7m 32s
Build RP2350 5	7m 42s
	2h 46m 38s

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