Remove Distribution and CURI.install compat shims

[ugexe]

These were only needed for panda compatibility, and because of a misuse of Distribution in roast that was changed to reflect the original intent of the test ( now fixed: Raku/roast@b82233a )

[Benabik]

Merging this broke ugexe/zef

zef (master)$ sw_vers 
ProductName:    Mac OS X
ProductVersion: 10.13.4
BuildVersion:   17E202
zef (master)$ nqp --version
This is nqp version 2018.05-33-gab16b6db9 built on MoarVM version 2018.05-51-gb74c0cbb0
zef (master)$ perl6 --version
This is Rakudo version 2018.05-48-gaf645c9f4 built on MoarVM version 2018.05-51-gb74c0cbb0
implementing Perl 6.c.
zef (master)$ git rev-parse HEAD
e5005f4ecf1dc2bc2a9c1f6c607d18b36f51efbc
zef (master)$ perl6 -Ilib bin/zef install .
===SORRY!=== Error while compiling /Users/benabik/dev/perl6/zef/lib/Zef/Distribution.pm6 (Zef::Distribution)
Method 'content' must be implemented by Zef::Distribution because it is required by roles: Distribution.
at /Users/benabik/dev/perl6/zef/lib/Zef/Distribution.pm6 (Zef::Distribution):5
zef (master)$ # Install pre-merge Rakudo
zef (master)$ perl6 --version
This is Rakudo version 2018.05-45-g18f9de61f built on MoarVM version 2018.05-51-gb74c0cbb0
implementing Perl 6.c.
brian@Hermes-BCG zef (master)$ perl6 -Ilib bin/zef install .
===> Testing: zef:ver<0.4.2>:auth<github:ugexe>
===> Testing [OK] for zef:ver<0.4.2>:auth<github:ugexe>
===> Installing: zef:ver<0.4.2>:auth<github:ugexe>

1 bin/ script [zef] installed to:
/usr/local/share/perl6/site/bin