alpkit: Remove base64 feature flag #43

Job	Run time
Run linters	24s
Test with --features shell-timeout	38s
Test with --features flate2-zlib --no-default-features	39s
Test with --features flate2-zlib-ng --no-default-features	51s
Test on x86_64-alpine-linux-musl (dynamically linked)	55s
Test on x86_64-unknown-linux-musl with Rust stable	28s
Test on x86_64-unknown-linux-musl with Rust nightly	55s
Test on x86_64-unknown-linux-musl with Rust 1.64	43s
Test on x86_64-unknown-linux-gnu with Rust stable	26s
Test on x86_64-apple-darwin with Rust stable	56s
Test on aarch64-unknown-linux-musl with Rust stable	1m 11s
Test on aarch64-unknown-linux-gnu with Rust stable	1m 12s
Test on armv7-unknown-linux-musleabihf with Rust stable	1m 13s
Test on powerpc64le-unknown-linux-gnu with Rust stable	1m 0s
Test on riscv64gc-unknown-linux-gnu with Rust stable	1m 7s
	12m 38s

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