rpm spec file: Remove ignored "BuildRoot" tag and "%clean" section

http://fedoraproject.org/wiki/Packaging:Guidelines#BuildRoot_tag says:

> Fedora (as of F-10) does not require the presence of the BuildRoot tag
> in the spec and if one is defined it will be ignored.

http://fedoraproject.org/wiki/Packaging:Guidelines#.25clean says:

> The %clean section is not required for F-13 and above.

(In my experiments when I removed the "%clean" section, "rpmbuild" still
executes "rm -rf %{buildroot}".)