Bug#838628: boost1.58: do not release with Stretch

[Emilio Pozuelo Monfort]

Source: boost1.58
Version: 1.58.0+dfsg-5.1
Severity: serious

We're releasing Stretch with boost 1.61. boost1.58 should be removed
from testing before the release. the three remaining rdeps in testing
have RC bugs.

Cheers,
Emilio