Remove portable syntax-case package from the bigloo directory.

Syntax-case is now standard in R5RS+ compliant schemes.
Remove portable syntax-case package from the gambit directory.

Syntax-case is now standard in R5RS+ compliant schemes, and available in
gambit via the -:s command-line option.
Remove portable syntax-case package from the mitscheme directory.

Syntax-case is now standard in R5RS+ compliant schemes.