Remove unused unexported variable.

The eraseUnderCursor variable is unused, it came in with the import of the
x/crypto/ssh/terminal package at d7a7210 but is not referenced here.

When this package is vendored, the unused unexported name trips warnings from
tools like staticcheck.

terminal.go

@@ -233,7 +233,6 @@ func (t *Terminal) queue(data []rune) {
 	t.outBuf = append(t.outBuf, []byte(string(data))...)
 }
 
-var eraseUnderCursor = []rune{' ', keyEscape, '[', 'D'}
 var space = []rune{' '}
 
 func isPrintable(key rune) bool {