[ntuple] Remove incorrect std::vector::reserve

This was added in commit d950e99 ("[ntuple] use RClusterPool in
RNTupleMerger") with no explicit mention, and I believe it's wrong
because every column yields exactly one page group that contains all
pages for that column.

tree/ntuple/v7/src/RNTupleMerger.cxx

@@ -274,8 +274,6 @@ void ROOT::Experimental::Internal::RNTupleMerger::Merge(std::span<RPageSource *>
             if (colRangeCompressionSettings != 0)
                sealedPageBuffers.resize(sealedPageBuffers.size() + pages.fPageInfos.size());
 
-            sealedPageGroups.reserve(sealedPageGroups.size() + pages.fPageInfos.size());
-
             std::uint64_t pageIdx = 0;
 
             // Loop over the pages