Remove ROOT_MOUNT_MASTER status

[vvb2060]

libsu/core/src/main/java/com/topjohnwu/superuser/internal/ShellImpl.java

Lines 164 to 166 in c9f1a6e

	STDIN.write(("readlink /proc/1/ns/mnt\n").getBytes(UTF_8));
	STDIN.flush();
	String s2 = br.readLine();

We can't be sure that readlink /proc/1/ns/mnt command will return content. If the standard output has no content (maybe in error output, or nothing at all), then readLine() will block.

https://github.com/topjohnwu/Magisk/blob/4856da1584c16f5c7387efcefe255c21a1978233/native/src/core/su/su.cpp#L38

    "  -t, --target PID              PID to take mount namespace from\n"

MagiskSU now supports setns, this check has lost its meaning. I don't think anyone really cares about state other than root, so this check can be removed. libsu should do as little as possible and leave any additional requirements to the developers themselves.

[vvb2060]

may be related #147 topjohnwu/Magisk#7021