[Rd] small issue with over-zealous clean.

[Hin-Tak Leung]

Noticed a problem for a while - tests/testit.Rd, tests/ver20.Rd are removed on "make clean" unintentionally.

This seems to come from a change in tests/Makefile.in, which adds the line:
   - at rm -f *.tar.gz *.Rd back in May 2012.

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commit c4d70254e7b7f9d7ed17faecfb3097195d852ddc
Author: ripley <ripley at 00db46b3-68df-0310-9c12-caf00c1e9a41>
Date:   Sun May 27 09:04:41 2012 +0000

    fix some issues seen by examining no-segfaults.Rout
    
    git-svn-id: https://svn.r-project.org/R/trunk@59451 00db46b3-68df-0310-9c12-caf00c1e9a41
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svn users probably don't notice files are unintentionally removed; git-svn does complain about version-controlled files being missing during rebase (i.e. re-applying local private patches to an updated upstream).

TIA.