logtail: don't strip verbose level on upload

For analysis of log spam.

Bandwidth is ~unchanged from had we not stripped the "[vN] " from
text; it just gets restructed intot he new "v":N, field.  I guess it
adds one byte.

Updates #1548

Change-Id: Ie00a4e0d511066a33d10dc38d765d92b0b044697
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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