remove unneeded dependency on BH (closes #80)

Author: kevinushey

DESCRIPTION

@@ -24,7 +24,6 @@ Suggests:
     RUnit,
     knitr,
     rmarkdown
-LinkingTo: BH (>= 1.60.0-1)
 SystemRequirements: GNU make, Windows: cmd.exe and cscript.exe, Solaris: g++ is required
 License: GPL-2
 URL: http://rcppcore.github.io/RcppParallel, https://github.com/RcppCore/RcppParallel