Drop unneeded client cert from metrics ServiceMonitor tlsConfig (#976)

The metrics server doesn't do client cert auth (auth is TokenReview/SAR
via bearerTokenFile), so cert/keySecret in the ServiceMonitor tlsConfig
are unused. This also breaks scraping through the OpenTelemetry
target allocator, which only forwards the collector a real cert/key
when TA mTLS is on, otherwise the key comes through as a literal
'<secret>' string and the scrape job fails.

ca and serverName stay since the server does present that cert.

Signed-off-by: Charles Cheng <chengxisheng777@gmail.com>