Remove deprecated -preview=markdown flag

The preview is now enabled by default

posix.mak

@@ -268,7 +268,7 @@ DDOC_VARS_PRERELEASE_VERBATIM=$(DDOC_VARS_PRERELEASE) \
 	DOC_OUTPUT_DIR="$W/phobos-prerelease-verbatim" \
 	STDDOC="$(PWD)/verbatim.ddoc"
 
-DDOCFLAGS=-c -o- -preview=markdown
+DDOCFLAGS=-c -o-
 
 ################################################################################
 # Ddoc binaries

spec/ddoc.dd

@@ -512,10 +512,6 @@ $(P
  +/
 ------------------------------------
 
-$(P
-$(EM Enable support for backticks, tildes and other code languages with the `-preview=markdown` compiler flag.)
-)
-
 $(H3 $(LNAME2 inline_code, Inline Code))
 
 $(P
@@ -568,10 +564,6 @@ best to avoid using it where practical.
 
 $(H3 $(LNAME2 headings, Headings))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 A long documentation section can be subdivided by adding headings. A heading is
 a line of text that starts with one to six `#` characters followed by whitespace
@@ -592,10 +584,6 @@ level. Headings may optionally end with any number of trailing `#` characters.
 
 $(H3 $(LNAME2 links, Links))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 Documentation may link to other documentation or to a URL. There are four
 styles of links:
@@ -689,10 +677,6 @@ normal link is used as the image's alt text.
 
 $(H3 $(LNAME2 lists, Lists))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 Documentation may contain lists. Start an ordered list with a number followed by
 a period:
@@ -762,10 +746,6 @@ after the list symbol:
 
 $(H3 $(LNAME2 tables, Tables))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 Data may be placed into a table. Tables consist of a single header row, a
 delimiter row, and zero or more data rows. Cells in each row are separated by
@@ -792,10 +772,6 @@ and `:`'s on both sides of the hyphens create a center-aligned column.
 
 $(H3 $(LNAME2 quotes, Quotes))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 Documentation may include a section of quoted material by prefixing each line of
 the section with a `>`. Quotes may include headings, lists, embedded code, etc.
@@ -822,10 +798,6 @@ Lines of text that directly follow a quoted line are considered part of the quot
 
 $(H3 $(LNAME2 hrules, Horizontal Rules))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 Create a horizontal rule by adding a line containing three or more asterisks,
 underscores or hyphens:
@@ -861,10 +833,6 @@ spaces:
 
 $(H3 $(LNAME2 text_emphasis, Text Emphasis))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
 A span of text wrapped in asterisks (`*`) is emphasized, and text wrapped in
 two asterisks (`**`) is strongly emphasized:
@@ -924,10 +892,6 @@ $(P
 
 $(H3 $(LNAME2 punctuation_escapes, Punctuation Escapes))
 
-$(P
-$(EM Enable this feature with the `-preview=markdown` compiler flag.)
-)
-
 $(P
     You can escape any ASCII punctuation symbol with a backslash `\`.
     Doing so outputs the original character without the backslash, except for