Configurations/10-main.conf: In the VC-common target, unquote $(CC)

Configurations/10-main.conf: In the VC-common target, unquote $(CC)

Some of the VC-common attributes have values that use `$(CC)`, wrapped with
quotes.  However, `Configurations/windows-makefile.tmpl` already quotes the
`CC` value, like this:

    CC="{- $config{CC} -}"

The interaction between that makefile variable and the attributes using
`$(CC)` wrapped with quotes is a command line with the quotes doubled.  For
example, the value of `$(CPP)` becomes `""cl""`.

Strangely enough, this appears to be tolerated, at least on some versions of
Windows.  However, this has been reported not to be the case.

This is fixed by removing the quotes in `Configurations/10-main.conf`,
making `Configurations/windows-makefile.tmpl` responsible for proper
quoting.

Fixes #18823

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18861)