Remove Validation section whose content can be found on the wiki

src/docs/chapter/user/20_Validation/00_section_header.adoc

...20_Validation/01_MC/Tutorial_Complete_Model_Checking.adoc

...er/20_Validation/01_MC/Tutorial_First_Model_Checking.adoc

...ocs/chapter/user/20_Validation/CBC/00_section_header.adoc

.../user/20_Validation/CBC/01_Constraint_Based_Checking.adoc

...ter/user/20_Validation/CBC/10_Bounded_Model_Checking.adoc

...er/user/20_Validation/CBC/20_Symbolic_Model_Checking.adoc

...Validation/CBC/Tutorial_Model_Checking_Proof_and_CBC.adoc

...ocs/chapter/user/20_Validation/CBC/ZZ_section_footer.adoc

...docs/chapter/user/20_Validation/MC/00_section_header.adoc

...hapter/user/20_Validation/MC/01_Consistency_Checking.adoc

.../user/20_Validation/MC/10_Distributed_Model_Checking.adoc

src/docs/chapter/user/20_Validation/MC/11_DMC.adoc

src/docs/chapter/user/20_Validation/MC/12_ParB.adoc

.../chapter/user/20_Validation/MC/20_LTL_Model_Checking.adoc

.../chapter/user/20_Validation/MC/30_Symmetry_Reduction.adoc

src/docs/chapter/user/20_Validation/MC/40_LTSmin

...er/20_Validation/MC/Tutorial_Directed_Model_Checking.adoc

...user/20_Validation/MC/Tutorial_Various_Optimizations.adoc

...docs/chapter/user/20_Validation/MC/ZZ_section_footer.adoc