Avoid explicit call to `EnableAutoLoading` for ROOT > 6.18

See https://sft.its.cern.ch/jira/browse/ROOT-10514
12 jobs for clemenci-master-patch-75226 in 32 minutes and 18 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #7312236
cvmfs
build:gcc8:opt

00:15:48

passed #7312238
cvmfs
build:gcc9:dbg

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passed #7312237
cvmfs
build:gcc9:opt

00:15:28

passed #7312239
cvmfs
build:gcc9:opt:python3

00:17:50

passed #7312240
check-formatting

00:01:57

 
  Test
passed #7312241
check-copyright

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passed #7312242
cvmfs
test:gcc8:opt

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passed #7312244
cvmfs
test:gcc9:dbg

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passed #7312243
cvmfs
test:gcc9:opt

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passed #7312245
cvmfs
test:gcc9:opt:python3

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passed #7312246
cvmfs
test_public_headers_build

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passed #7312247
test-python3

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