Purge xbus in BaseSubsystem #853

Job	Run time
riscv-arch-test	0s
emulator (DefaultSmallConfig)	13m 3s
riscv-tests (DefaultConfig)	19m 24s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultC...	21m 31s
riscv-tests (DefaultBufferlessConfig)	19m 31s
emulator (DualBankConfig)	13m 5s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultR...	19m 58s
riscv-tests (DefaultRV32Config)	19m 34s
emulator (DualChannelConfig)	13m 5s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	18m 4s
riscv-tests (TinyConfig)	13m 16s
emulator (DualChannelDualBankConfig)	14m 4s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	17m 17s
riscv-tests (DefaultFP16Config)	14m 11s
emulator (RoccExampleConfig)	13m 16s
emulator (Edge128BitConfig)	13m 16s
emulator (Edge32BitConfig)	13m 31s
emulator (QuadChannelBenchmarkConfig)	13m 21s
emulator (EightChannelConfig)	13m 48s
emulator (DualCoreConfig)	13m 44s
emulator (MemPortOnlyConfig)	13m 12s
emulator (MMIOPortOnlyConfig)	12m 41s
emulator (CloneTileConfig)	14m 29s
emulator (HypervisorConfig)	13m 8s
	5h 50m 29s

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