Remove needless lock from log.cpp/ElapsedSeconds

[Kerdek]

Static local variable start was guarded by a mutex, but all concurrent accesses are reads.

[mmp]

Indeed, this is unnecessary, though looking at the commit logs, it looks like I added this to silence bogus warnings from helgrind. So I think it's worth keeping it in the interests of making helgrind use more pleasant, when it's necessary...