common: remove duplicated empty lines

CONTRIBUTING.md

LICENSE

README.md

doc/Makefile

doc/daxio/daxio.1.md

doc/libpmem/libpmem.7.md

doc/libpmem/pmem_flush.3.md

doc/libpmem/pmem_is_pmem.3.md

doc/libpmem/pmem_memmove_persist.3.md

doc/libpmem2/libpmem2.7.md

doc/libpmem2/pmem2_config.3.md

doc/libpmem2/pmem2_errormsg.3.md

doc/libpmem2/pmem2_mapping.3.md

doc/libpmemblk/libpmemblk.7.md

doc/libpmemblk/pmemblk_bsize.3.md

doc/libpmemblk/pmemblk_create.3.md

doc/libpmemblk/pmemblk_ctl_get.3.md

doc/libpmemblk/pmemblk_read.3.md

doc/libpmemblk/pmemblk_set_zero.3.md

doc/libpmemlog/libpmemlog.7.md