README.md: remove old warning about ext4 encryption on removable media #247

Job	Run time
Run command-line interface tests	1m 7s
Build (32-bit)	42s
Build (Go 1.18)	49s
Build (Go 1.19)	45s
Run integration tests	55s
Build (Go 1.20)	31s
Generate, format, and lint	52s
	5m 41s

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