Remove outdated info from dmd manual

[dkorpel]

A Google Summer of Code contributor came across build instructions here, which are outdated. Upon closer inspection, lots of other info on this page is out of date as well.

• omf support is removed, so the Digital Mars C compiler (dmc) is useless for D now
• dman, obj2asm, dumpobj, optlink, shell, make are not part of a dmd release anymore
• the samples folder is removed

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon. Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[ntrel]

/tmpfs/dautotest-work/repo/dlang.org/web/dmd-osx.html:113: [RAW MACRO LEAKAGE] $(D_S dmd - Mac OS X D Compiler,
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-windows.html:113: [RAW MACRO LEAKAGE] $(D_S dmd - Windows D Compiler,
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-osx.html:134: [RAW MACRO LEAKAGE]     $(OL
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-windows.html:134: [RAW MACRO LEAKAGE]     $(OL
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-freebsd.html:113: [RAW MACRO LEAKAGE] $(D_S dmd - FreeBSD D Compiler,
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-freebsd.html:134: [RAW MACRO LEAKAGE]     $(OL
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-linux.html:113: [RAW MACRO LEAKAGE] $(D_S dmd - Linux D Compiler,
/tmpfs/dautotest-work/repo/dlang.org/web/dmd-linux.html:134: [RAW MACRO LEAKAGE]     $(OL
8 errors found. Exiting.