drop `opts.timer` option passed to Span or Transaction creation

[trentm]

Delete any opts.timer option passed to Span or Transaction creation. While I don't expect anyone to have ever used this option, it is theoretically a breaking change, so should be left to a major ver bump.

#2939 is adding a deprecation warning for passing in opts.timer to Span or Transaction creation. It is an internal option used to pass to the internal GenericSpan based class, but users should never want or need to use this option.

[trentm]

FWIW the current warning looks like this:

> var s2 = apm.startSpan('s2', {timer: {}})
undefined
> (node:72351) [ELASTIC_APM_SPAN_TIMER_OPTION] DeprecationWarning: specifying the `timer` option to `new Span()` was never a public API and will be removed

[trentm]

Closed by #3537 on the "dev/4.x" branch, which will become the 4.x active branch soonish.