Revert the hconsShape workaround.

It is no longer needed in Scala 2.10.4 and 2.11, and it prevented some
uses of HLists.

Test in JdbcMapperTest.testSingleElement (did not compile with the
workaround in place). Fixes issue #728.
Merge pull request #792 from slick/tmp/issue-728

Revert the hconsShape workaround.