storage, db, mixer, command: remove more bogus "pure" attributes

This commit is similar to 788e3b31e1,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

src/pcm/Resampler.hxx

@@ -63,11 +63,12 @@ public:
 	/**
 	 * Resamples a block of PCM data.
 	 *
+	 * Throws std::runtime_error on error.
+	 *
 	 * @param src the input buffer
 	 * @return the destination buffer (will be invalidated by
 	 * filter_close() or filter_filter())
 	 */
-	gcc_pure
 	virtual ConstBuffer<void> Resample(ConstBuffer<void> src) = 0;
 };