Purge xbus in BaseSubsystem #850

Job	Run time
emulator (DefaultSmallConfig)	12m 59s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultC...	23m 44s
riscv-arch-test	0s
emulator (DualBankConfig)	13m 24s
riscv-tests (DefaultConfig)	19m 39s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultR...	22m 5s
emulator (DualChannelConfig)	13m 17s
riscv-tests (DefaultBufferlessConfig)	19m 33s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	20m 28s
emulator (DualChannelDualBankConfig)	13m 36s
riscv-tests (DefaultRV32Config)	21m 43s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	19m 25s
emulator (RoccExampleConfig)	13m 9s
riscv-tests (TinyConfig)	15m 41s
emulator (Edge128BitConfig)	13m 12s
riscv-tests (DefaultFP16Config)	14m 6s
emulator (Edge32BitConfig)	13m 11s
emulator (QuadChannelBenchmarkConfig)	13m 22s
emulator (EightChannelConfig)	13m 34s
emulator (DualCoreConfig)	13m 39s
emulator (MemPortOnlyConfig)	12m 55s
emulator (MMIOPortOnlyConfig)	12m 53s
emulator (CloneTileConfig)	14m 37s
emulator (HypervisorConfig)	12m 59s
	6h 3m 11s

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