Remove header words from number of cluster estimate

The header word is now not counted anymore in the estimate for the number of FTClusters.

• [2018-06-06 00:13] Validation started with lhcb-clang-test#975
• [2018-06-06 00:14] Validation started with lhcb-head#1876
• [2018-06-06 00:14] Validation started with lhcb-lcg-dev4#547
• [2018-06-06 00:15] Validation started with lhcb-tdr-test#173
• [2018-06-06 00:16] Validation started with lhcb-lcg-dev3#547
• [2018-06-06 00:18] Validation started with lhcb-gaudi-head-py3#176
• [2018-06-06 00:18] Validation started with lhcb-gaudi-head#1890

merged

mentioned in commit 7a06b5a7