cmd/godoc: make it possible to copy code without line numbers

[gopherbot]

by feedback.test.account:

https://feedback.corp.google.com/#/Report/1007452076

Description:
Could you fix it so that when I copy and paste the line numbers aren't copied? GitHub
does this. 

UI language: en
Detected language: en

[adg]

Comment 1:

For example: http://golang.org/src/pkg/crypto/subtle/constant_time.go

Labels changed: added godoc.

Status changed to Accepted.

[bjkail]

Comment 3:

There is a "View as plain text" link at the bottom (which isn't to say that enhancing
the html view to omit line numbers isn't also useful).

[rsc]

Comment 4:

Labels changed: added go1.2maybe.

[rsc]

Comment 5:

Labels changed: added feature.

[robpike]

Comment 6:

Status changed to Duplicate.

Merged into issue #4656.