eng: remove PublicSign windows-only restriction

[mgravell]

fix #2961

problem: linux build does not strong-sign (or rather: it public-signs, which is like delay-sign but: looser)

The cause is explicit in the csproj; this was added 7 years ago; I assume this was due to a glitch in .NET Core 2.x (as-was at the time), on non-Windows, but: checking locally: it works fine now

> sn -v bin/Release/net8.0/StackExchange.Redis.dll
Mono StrongName - version 6.12.0.199
StrongName utility for signing assemblies
Copyright 2002, 2003 Motus Technologies. Copyright 2004-2008 Novell. BSD licensed.

Assembly bin/Release/net8.0/StackExchange.Redis.dll is strongnamed.